Stable isotopes (13C, 15N, 17O and 18O, etc.) of carbon, nitrogen and oxygen, etc. are used as a tracer in the fields of natural science and medical care. As an enrichment method of these isotopes which exist in only little amounts in nature, there is a cascade process which uses a plurality of distillation columns.
Cascade means connecting a plurality of distillation columns in series. In order to continuously concentrate a specific component in raw materials, the component concentrated in a distillation column is further concentrated in a latter distillation column, and again concentrated in a further latter distillation column. That is, a continuous distillation process is performed by a plurality of distillation columns. In this respect, a cascade process is different from a process seen in a general chemical process, which combines a plurality of distillation columns in which a component to be concentrated is different from the others.
A cascade process is a technique which is mainly used in the field of isotope enrichment. This cascade process enables enrichment by distillation for a structural isomer or an isotope or isotopologue which has a separation factor (or relative volatility) of almost 1, requires a very large number of theoretical plates, and is difficult to be separated.
Hereinafter, an example of a conventional cascade process is described.
In a cascade process, as a method of exchanging a material between adjacent distillation columns, i.e. a connection method, there are methods shown in FIG. 4 to FIG. 7.
The distillation apparatus shown in FIG. 4 is an example of the simplest distillation cascade. This example of a distillation apparatus is composed of a distillation column group in which 6 distillation columns D1 to D6 are connected in series. The distillation columns D1 to D3 constitute a stripping section, and the distillation columns D3 to D6 constitute an enriching section. In the distillation columns D1 and D2, the condensers C1 and C2 are provided on the tops thereof. In the distillation column D4 to D6, the reboilers R4 to R6 are provided on the bottoms thereof. In the distillation columns D3, the condenser C3 is provided on the top thereof, and the reboiler R3 is provided on the bottom thereof.
A feed gas F is fed into the distillation column D3. Then, the desired component is concentrated and withdrawn from the bottom part of the distillation column D6 as a product P, and the remainder is withdrawn from the top part of distillation column D1 as waste components W.
The distillation load is the largest in the distillation column D3 to which the feed gas F is fed. The load becomes gradually small toward the last column D6 in the enriching section and toward the first column D1 in the stripping section (that is, the column diameter becomes small).
In the example of this apparatus, the returns of gases from the last column D6 to the fifth column D5, from the fifth column D5 to the fourth column D4, . . . , and from the second column D2 to the first column D1 are performed by using pressure differences. Therefore, the pressure of a distillation column needs to be higher toward the last column D6 from the first column D1. As a result, a separation factor (relative volatility) also becomes small, thereby resulting in a disadvantage with respect to distillation efficiency.
Also, when liquid pumps P1 to P5 are used to flow the liquid of a distillation column to the latter column, liquid is accumulated in the liquid pumps P1 to P5. Therefore, the liquid hold-up over the whole apparatus is increased, which is disadvantageous in that startup time is extended. Also, when a liquid pump is used in cryogenic distillation, heat inleak is increased, and thus, there is a disadvantage in this respect.
The distillation apparatus shown in FIG. 5 is another example of conventional art. The distillation columns thereof have almost the same features as the example of the apparatus shown in FIG. 4. In this example, pressures at the tops of all the distillation columns D1 to D6 are the same. Thus, it is possible to prevent pressures from increasing toward the last distillation column D6 and to prevent the separation factor from becoming smaller. However, this apparatus requires pressurizing devices such as blowers B1 to B5 are required for returning gases to the former distillation columns respectively, which is disadvantageous in reliability of the apparatus. Also, the disadvantages regarding the use of the liquid pumps P1 to P5 are not solved.
The example shown in FIG. 6 is an example of conventional art which is the developed version of the apparatus shown in FIG. 5. In a similar manner to the apparatus shown in FIG. 5, pressures at tops of all distillation columns D1 to D6 are the same. All the distillation columns D1 to D6 are equipped with the condensers C1 to C6 and the reboilers R1 to R6, and a gas is fed into the latter distillation columns by pressure differences between adjacent distillation columns (corresponding to pressure drop in the case where pressures at tops are the same).
In this apparatus, liquid pumps are not used, and thus, it is possible to decrease liquid hold-up. However, the disadvantage is not solved in that the pressurizing device such as blowers B1 to B5 are required to return gases.
The distillation apparatus shown in FIG. 7 is a modified example of the apparatus shown in FIG. 6. In this example, the blowers used for returning gases are omitted, and instead, the liquid-return lines Q1 to Q5 are respectively used to store the liquid obtained by liquefaction in condensers C2 to C6 and to return these liquid to the former distillation columns by the liquid head pressure (liquid head) therein.
In this apparatus, both the feeding and returning devices do not require a rotary machine such as a pump or a blower. Thus, reliability of this apparatus is improved, and liquid hold-up in a liquid-return line can be minimized, which is advantageous in that startup time is shortened. Also, it is advantageous that pressures of all the distillation columns D1 to D6 are low because it contributes to the increase in the separation factor. However, a condenser and a reboiler are required for each distillation column, which is disadvantageous from the view of the apparatus cost.
As described above, for the separation of isotopes, isotopologues or structural isomers, a plurality of distillation columns is connected in the form of a cascade, which is operated as if these were one distillation column. Thus, it is necessary to surely feed one or both of gas and liquid from a former distillation column to the latter distillation column through the connections between adjacent distillation columns D1 to D6. For example, in the distillation apparatus shown in FIG. 7, a part of gas existing in the bottom of the former distillation column or the vicinity thereof, or in the exit of the reboiler is surely fed to the latter distillation column through a feeding line, and is liquefied in the condenser so as to become a part of a falling liquid in the latter distillation column.
Patent Literature 1 discloses one example of the distillation apparatus shown in FIG. 7.
However, in a case where packed columns packed with structured packings are used in a distillation apparatus as shown in FIG. 6 or FIG. 7 or in the distillation apparatus disclosed in Patent Literature 1, pressure differences (driving force for flow) in gas-feeding lines are very small, and thus, the flow may become unstable.
Moreover, when the connection point between the gas-feeding line and the latter distillation column is set at the gas pipe (the entrance of the condenser) of the main body or the upper part of the distillation column, the inflow of the feed gas is interrupted by the gas flow, specifically fluctuation and dynamic pressure of the gas flow, in the gas-feeding line on the side of the latter distillation column, and there is the problem that it may be impossible to stably feed the gas. Also, the feed gas joins the top-gas of the latter distillation column, and then, the mixed gas is liquefied. A part of the liquid is returned to the former distillation column without reflux to the latter distillation column, and thus, there is the problem that it is impossible to efficiently connect the distillation columns.
This problem will be described in details with reference to FIG. 8. FIG. 8 is the same as FIG. 1 of Patent Literature 1, and illustrates the substantially same features as FIG. 7.
In the distillation apparatus shown in FIG. 8, the gas is fed from the distillation column D1 to the distillation column D2 through the gas-feeding line 12. A part of the gas at the exit of the reboiler 6 of the distillation column D1 flows to the distillation column D2 as the feed gas 104, and joins the top-gas 106 which flows in the top-gas pipe 28 of the distillation column D2 through the flow rate regulating valve 12v. The mixed gas flows into the condenser 7. The liquid generated by the condensation in the condenser 7 is refluxed to the top of the distillation column D2, and a part of the liquid is returned to the distillation column D1 through the return line 14 as the returning liquid 107.
In this distillation apparatus, the flow rate of the top-gas 106 is generally several to tens times as high as that of the feed gas 104. For this reason, the join of the both gases occurs in the manner that a small amount of the feed gas 104 flows into a large amount of the top-gas 106, and this inflow of the feed gas 104 may be problematic due to the dynamic pressure, etc. of the flow of the top-gas 106. This effect on the flow is significant specifically in the case where a packed column packed with structured packing is used as the respective distillation columns D1 and D2 because the driving force of the feed gas 104 is very small and within a range from several kPa to ten and several kPa.
Moreover, the feed gas 104 joins the top-gas 106, the both gases are mixed together, and the mixed gas is condensed in the condenser 7. Then, a part of the liquid is returned to the distillation column D1 through the return line 14 as the returning liquid 107. As a result, a part of the feed gas 104 is returned to the distillation column D1 as a part of the returning liquid 107 without being fed to the distillation column D2.
Although FIG. 8 illustrates that the feed gas 104 joins the top-gas 106 in the pipe, the same problem occurs in the case where the gas-feeding line is directly connected to the upper part of the distillation column as shown in FIG. 7.